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The carbon cycle is a complex thing. There is carbon in the air (carbon dioxide), carbon in plants and animals, dissolved carbon in the sea and carbon in the soil that is constantly circulating to and from. Elevated levels of atmospheric carbon dioxide may accelerate carbon cycling and soil carbon loss in forests, as found in new research led by an Indiana University biologist. The new evidence supports an emerging view that although forests remove a substantial amount of carbon dioxide from the atmosphere, much of the carbon is being stored in living woody biomass rather than as dead organic matter in soils.
Soil carbon is the generic name for carbon held within the soil, primarily in association with its organic content. Soil carbon is the largest terrestrial pool of carbon. Soil carbon plays a key role in the carbon cycle and thus is important in global climate models.
Richard P. Phillips, lead author on the paper said that after nearly two decades of research on forest ecosystem responses to global change, some of the uncertainty has been lifted about how forests are storing carbon in the wake of rising carbon dioxide levels.
“It’s been suggested that as trees take up more carbon dioxide from the atmosphere, a greater amount of carbon will go to roots and fungi to acquire nutrients, but our results show that little of this carbon accumulates in soil because the decomposition of root and fungal detritus is also increased,” he said.
Carbon stored in soils, as opposed to in the wood of trees, is desirable from a management perspective in that soils are more stable over time, so carbon can be locked away for hundreds to thousands of years and not contribute to atmospheric carbon dioxide increases.
The research was conducted at the Duke Forest Free Air Carbon Dioxide Enrichment site in North Carolina. At this site, mature loblolly pine trees were exposed to increased levels of carbon dioxide for 14 years, making it one of the longest-running carbon dioxide enrichment experiments in the world. Researchers were able to calculate the age of the carbon cycling through the soil by growing roots and fungi into mesh bags that contained uniquely labeled soils. The soils were then analyzed for their organic composition.
The authors also report that nitrogen cycled faster in this forest as the demand for nutrients by trees and microbes became greater under elevated CO2.
“The growth of trees is limited by the availability of nitrogen at this site, so it makes sense that trees are using the extra carbon taken up under elevated CO2 to prime microbes to release nitrogen bound up in organic matter,” Phillips said. “What is surprising is that the trees seem to be getting much of their nitrogen by decomposing root and fungal detritus that is less than a year old.”
The two-fold effects of microbial priming, where microbes are stimulated to decompose old soil organic matter via an increase in new carbon and other energy sources, and the faster turnover of recently fixed root and fungal carbon, are enough to explain the rapid carbon and nitrogen cycling that is occurring at the Duke Forest site.